This invention relates to high temperature airfoils for axial flow rotary machines. Such airfoils conventionally have internal cooling chambers through which a cooling medium flows during operation of the machine. One effecitve technique for fabricating airfoils with internal chambers is described in U.S. Pat. No. 3,029,485 to McCormick entitled "Method of Making Hollow Castings". In McCormick, a core mold extending in the spanwise direction through the airfoil is supported during the casting process, outside of the airfoil at both the root and tip ends. Supporting the core mold from both ends decreases the likelihood that the core will shift from physical handling or from thermal shocks during the casting procedure. Collaterally, the opening left in the airfoil tip when the core mold is removed facilitates inspection of the interior for casting flaws. Having fabricated the airfoil tip with an opening, the opening must be closed to prevent the leakage of cooling air from the internal chamber during operation. Tip caps are commonly used to close the opening.
One tip cap structure is shown in U.S. Pat. No. 2,779,565 to Bruckmann entitled "Air Cooling of Turbine Blades." In Bruckmann, the tip cap is welded to the tip of the airfoil. In modern, high temperature engines, weld materials having sufficient yield strength often have inherently low fatigue strength. This low fatigue strength may precipitate low cycle fatigue failure of the weld and may result in the leakage of cooling air from the internal chamber.
U.S. Pat. No. 3,982,851 to Andersen et al entitled "Tip Cap Apparatus", U.S. Pat. No. 4,010,531 to Andersen et al entitled "Tip Cap Apparatus and Method of Installation" and U.S. Pat. No. 4,073,599 to Allen et al. entitled "Hollow Turbine Blade Tip Closure" are of interest for their contemporaneous showings of structures alternative to those of the present invention. These structures locally support the tip cap but do not provide continuous engagement between the tip cap and the airfoil structure. These structures rely on the deposition of brazed material to prevent the leakage of air from the interior of the airfoil. High speed inertial forces acting on the tip cap in regions without continuous engagement cause the tip cap to pull away from the adjacent structure. The cooling medium in the internal chamber leaks between the cap and the adjacent structure, and decreased cooling effectiveness results.
The need to produce energy efficient machines has grown in recent years because of increased fuel costs and limited fuel supplies. As a result, research efforts are being directed toward improving the cooling effectiveness and structural integrity of airfoil structures.